The environment of the intestinal lumen affects the activity of Crohn's disease, and therefore luminal factors are involved in the pathogenesis of chronic disease. This phenomenon is evident in patients receiving an elemental diet, where changes in the luminal environment often result in disease remission. However, the mechanisms linking the luminal environment to mucosal immune activity have not been fully explored at a molecular level. This proposal will examine the hypothesis that the luminal environment influences mucosal immune responses by regulating the expression of enterocyte surface molecules that interact with T cells. In an immune response, exogenous antigens are presented to T lymphocytes by class II MHC molecules. As class II MHC expression in the small intestinal epithelium is affected by diet, the role of the epithelium as a link between the luminal environment and the mucosal immune system will be examined in the mouse. In addition, the expression of costimulatory molecules in the epithelium, being differently regulated from class II MHC, will be studied. First, the molecular mechanisms by which an elemental diet affects class II MHC expression in the epithelium during development will be examined. The expression of class II MHC and invariant chain will be correlated with the initiation of their transcription by nuclear-run on. The effect of diet on the binding of nuclear factors to the invariant chain promoter will be studied, and compared to the effects of microenvironmental factors in a rodent enterocyte cell line. Second, the effect of elemental diets on the expression of costimulatory molecules (B7-1 and -2) in' epithelial cells, and class 11 MHC molecules will be studied l) in mice infected by reovirus and 2) in the C3H/HeJ Bir mouse IBD model. Protein expression (by immunocytochemistry and Western Blots) and mRNA accumulation (by Northern blots and quantitative PCR) will be examined in the epithelium and lamina propria of large and small intestine. Third, the effect of an elemental diet on the relationship between class II MHC, costimulatory molecules (epithelial) and T cell activation (lamina propria) will be examined during the progression of inflammation caused by reovirus infection and in the C3H/HeJ Bir mouse IBD model. T cell markers (CD25, CTLA-4 and CD28); intraepithelial lymphocytes; and T cell subsets will be examined under dietary changes in the weanling mouse, reovirus infected mouse and in the IBD mouse models. It will be possible to examine the effects of changes in the luminal environment to the mucosal immune system through molecular events occurring in the intestinal epithelium.